Traveling wave tube helix support structure



O United States Patent 11113,551,729

[ 72] Inventor Charles E. Bradford 56] References Cited gigg UNITED STATES PATENTS 3 9f; 25 969 2,863,085 12/1958 Robertson 315/15 E la Dec 29 1970 3,286,120 11/1966 Peterson et al. 315/3.6

[73] Assignee Bell Telephone Laboratories, Incorporated Primary Examiner-Herman Karl Saalbach Murray Hill, NJ. Assistant Examiner-Saxfield Chatmon, Jr. a corporation of New York Attorneys-R. J. Guenther and Arthur J. Torsiglieri 54] TRAVELING WAVE TUBE HELIX SUPPORT ABSTRACT: In a traveling-wave tube using a periodic permanent magnet focusing system, a plurality of helix turns nearest the collector are unsupported by the dielectric support rods. This increases the tolerable electron impingement of the helix by reducing the heat transfer to the support rods and resultant contamination by out-gassing.

f PATE mEu-ntczsmc mm mm J 1 /NVENTOR C. E. BRADFORD.

ATTORNEY WAVE TUBE HELIX SUPPOR STRUCTURE BACKGROUND OF THE INVENTION TRAVELING tron beam fiow, although this is not *necessarilythecase.

Traveling-wave tubes-may also be usedas oscillators; that is, as microwave frequency generators.

The electron beam is normally constrainedto flow within the helix by an externally applied magnetic field. As is described, for example, in C. E. Bradford, US. Pat. No. 3,355,622, issued Nov.- 28, l967,the weight and bulk of the external'magnet system can be substantially reduced by using an array of annular permanent magnets arranged to establish a spatially alternating magnetic field along the electron beam path. The conductive helix is typically connected at opposite ends to a cylindrical input coupler and an output" coupler and is supported by three dielectric support rods thatmay conveniently be mounted on the input and output couplers.

There is a natural tendency of the electron beam diameter, when radio frequency input power is being amplified to high amplitudes, to expand near the downstream or collector end of the helix because of the removal of the kinetic energy from the beamby the helix wave; this is especially'true when the R- F- input power is being amplified to a high amplitude. Beam expansion is particularly pronou need if periodic focusing is used because, when the reduced velocity electrons fall out of synchronism with the spatially alternating magnetic field, a condition is caused known as beaml ripplen The expanded beam diameter may result in localized electron impingement on the downstream end of the helix with concomitant local (spot) heating to high temperatures. With excess electron impingement, the support rods will be heated to such an extent that first, their dielectric constant changes significantly;

' and secondly, gases are driven from the'rods. The change of dielectric constant changes the helix wave velocity to throw it out of synchronism with the electron beam, thereby reducing device efficiency, while the gases driven 'frorn'the rods create ion noise'and eventually contaminate the cathode.

One way of avoiding the problem is simply to make the helix diameter sufficiently large. This, however, reduces interaction efficiency and attainable amplification. One could also make the support rods of a thermally conductive material such a beryllia, but this approach is expensive for a variety of reasons, such as the brittleness of the material.

SUMMARY OF THE INVENTION In accordance with the invention, the problems associated with excessive support rod heating are substantially reduced or eliminated by separating the helix turns nearest the collector from the support rods. Normally, each helix turn is bonded, as by glazing, to all of the support rods, and this is trons, the resultant heat is not transferred directly to the support rods, a greater portion of heat is dissipated by radiation,

and intense local heating of the support rods is substantially eliminated. For the reasons given before, this increases the efficiency and' lifetime of the entire traveling wave tube.

These and other'objects, features, and advantages of the invention will be better understood from a consideration of the following detailed description taken in conjunction with the accompanying drawing.

DRAWING DESCRIPTION FIGS. 1A and 18 together are a sectional view of a traveling-wave tube in accordance with an illustrative embodiment of the invention;

FIG. 1C illustrates how FIGS. 1A and 1B are intended to fit together;

FIG. 2 is an enlarged view of part of the traveling-wave tube of FIG. 1; and

FIG. 3 is a view taken along lines 3-3 of FIG. 2.

DETAILED DESCRIPTION Referring now to FIG. 1, there is shown a traveling-wave tube, the purpose of which is to amplify electromagnetic wavestransmitted by an input waveguide 11, an input coupling member 12, a helix 13, an output coupling member 14, and an output waveguide 15. An electron beam is form ed and projected along the helix axis by an electron gun comprising a cathode l6, beam forming electrode 17, and accelerating anode 18. The beam is maintained within a vacuum by an evacuated envelope 20 and is collected by a collector electrode 21. An array of annular permanent magnets 22 surrounding the envelope focuses the beam and constrains it to flow along the central axis to the collector in accordance with the principles of periodic focusing. An array of pole pieces 23 maintains the desired spatial periodicity of the magnetic field in accordance with the principles described in C. E. Bradford US. Pat. No. 3,355,622, issued Nov. 28, 1967, and assigned to Bell Telephone Laboratories, Incorporated.

In accordance with well-known principles of traveling-wave tube operation, the electron beam velocity and the helix pitch are appropriately interrelated so that the wave on the helix travels in approximate synchronism with the beam. Axial fields associated with the wave modulate component electrons of the beam to form space-charge waves which are coupled with the helix as they travel toward the collector. The coupled waves abstract drift velocity energy from the beam and convert it to microwave energy such that the wave appearing at the output waveguide is an amplified form of that introduced by the input waveguide. 3

As is seen more clearly in FlGS. 2 and 3, the helix 13 is supported by three axially extending dielectric rods 25, each bonded to an extending between the input and output couplers l2 and 14. As was mentioned before, there is a tendency for the rods to become excessively heated due to electron impingement on the downstream end of the helix. Thiseffeet is especially pronounced in a periodically focused electron beam because the magnetic field spatial alternations create a condition known as beam ripples, manifested by predictable periodic locations of expanded beam diameter. As the beam velocity is reduced, due to high R-F fields, the ripple is increased, and unless the helix diameter is unduly large, a maximum-diameter region of the beam impinges the helix near the downstream end. With a normally'constructed helix assembly, the resultant heat would betransmitted directly to the-support rods and, for the reasons given before, device efficiency and lifetime would be limited.

In accordance with the invention, tube efficiency and lifetime are increased by separating the last l0 turns of the helix from the support rods. This is preferalby done by cutting away a portion of the support rods and the final 10 turns of the helix. The gap constitutes a vacuum insulation between the helix and the support rods and prevents heat from being directly transferred between those helix turns and the support rods. The majority of the helix turns each contact the support rods and are each glazed to all of the support rods as is customary in the art.

The tube of FIG. 1. in which the invention has been used, has the following characteristics:

Ration of beam diameter to helix diameter 5 Average helix diameter inch 1023 Frequency of operation gigahertz 5. 925-6. 425 Beam current -amps- 059 Helix length inches- 8. 04 Helix turns per inch 24. 09 Helix synchronous voltage volts 39. 50 Peak focusing field -gauss- 1070 Magnetic field period inch 606 Cathode diameter do 246 Collector voltage volts- 1800 Anode voltage do 4350 Heater voltage do 6. 8

A solenoid coil 28 surrounding the electron gun provides a magnetic field for matching to the field provided by magnets 22 and suppresses buildup of beam noise. A shielding member 29 surrounding the gun prevents magnetic fringing fields from extending into the electron gun. A cylindrical conductive member 30 defines with the shielding member 29 and coupling member 32, a radial waveguide 31 that acts as a choke at the operating frequency and thereby forces the input waves to propagate along the helix as desired. An annular aluminum casting provides structural stability to the assembly. With these parameters, it can be shown that the tube gives relatively high efficiency interaction while avoiding the problems that would normally arise from excessive heating of the support rods. The tube parameters, particularly the ratio of beam diameter to helix diameter, are designed such that significant beam impingement occurs only on those helix turns separated from the support rods. As used herein, the term helix" denotes any twisted wire slow wave structure, and is not intended to be used in the mathematical or geometric sense.

The specific embodiment shown described is intended to be only illustrative of the principles involved. Various other embodiments and modifications may be made by those skilled in theart without departing from the spirit and scope of the invention.

lclaim:

1. in a traveling-wave tube of the type comprising means for forming and projecting a beam of electrons, means for propagating high frequency energy in interacting relation with the beam comprising an elongated conductive helix extending coaxially with the beam and arranged such that the beam enters an upstream end of the helix and exits from a downstream end, and means for supporting the helix comprising a plurality of dielectric support rods each parallel to the helix axis and contacting the helix, the improvement wherein: a plurality of helix turns immediately adjacent the downstream end of the helix are out of contact with the support rods and separated from the support rods by a vacuum gap, thereby inhibiting heat transfer from said plurality of turns to the support rods.

2. The improvement of claim 1 wherein:

approximately percent of the helix turns contact the support rods; and

approximately 5 percent of the helix turns are out of contact with the support rods.

3. In a traveling-wave tube of the type comprising means for forming and projecting a beam of electrons from a cathode to a collector, means for focusing the electron beam, and means for propagating high frequency energy in interacting relation with the beam comprising a conductive helix extending from an input coupler member near the cathode to an output coupler member near the collector, the improvement comprising:

means for supporting the helix comprising a plurality of dielectric support rods, each mounted at one end on the input coupler member and at the opposite end on the outa minor part of each support rod being located immediately adjacent the output coupler member and having a crosssectional area that is smaller than that of the major part; and

a plurality of helix turns being coextensive with the minor part of the support rods and being unconnected to the support rods, thereby inhibiting heat transfer to the support rods.

4. The improvement of claim 3 wherein: said focusing means comprises a periodic permanent magnet focusing system.

5. The improvement of claim 4 wherein:

the periodic permanent magnet focusing results in electron beam ripple, a ripple amplitude during operation growing with distance in the direction of the collector; and

the ratio of the average beam diameter to the helix diameter being such that a maximum amplitude portion of the beam impinges on part of said plurality of helix turns coextensive with the minor parts of the support rods. 

